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Nonlinearity, frequency stability and device-to-device variability in nano-contact spin torque oscillators with grainy thin films
KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.ORCID iD: 0000-0003-1271-1814
Göteborgs universitet.
NanOsc AB.
KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

In nano-contact spin torque oscillators with a frequency range of 10-65 GHz, the propagating spin wave mode attracts interest due both to its high frequency stability and prospective use in magnonic devices. Its dependence of the frequency on the bias current however displays device-to-device variability on the order of several hundred MHz, with device specific nonlinearities that can be either continuous or discontinuous and have negative impact on the frequency stability. A model for this behavior is however still lacking. By using micromagnetic simulations, we investigate the impact of imperfections in the spin wave-carrying free magnetic layer and find that nonlinearities can be created when the propagating spin wave is reflected back to the active region. The oscillation then self-locks at the frequency of the resonant wavelength, resulting in a standing spin wave pattern. Simulations including nine randomly generated film structures with 30 nm-sized grains and exchange-reduced inter-grain boundaries give qualitative and partially quantitative agreement with experimental measurements. The results point out the spin wave-reflecting grain boundaries as a source of device nonlinearity, manufacturing variability and frequency destabilization.

Keyword [en]
Spin torque, spin waves, magnetization dynamics, thin films, microstructure, microwaves, phase noise
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:kth:diva-188544OAI: oai:DiVA.org:kth-188544DiVA: diva2:936364
Funder
Swedish Research Council
Note

Qc 20160616

Available from: 2016-06-13 Created: 2016-06-13 Last updated: 2016-06-20Bibliographically approved
In thesis
1. Microwave Frequency Stability and Spin Wave Mode Structure in Nano-Contact Spin Torque Oscillators
Open this publication in new window or tab >>Microwave Frequency Stability and Spin Wave Mode Structure in Nano-Contact Spin Torque Oscillators
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The nano-contact spin torque oscillator (NC-STO) is an emerging device for highly tunable microwave frequency generation in the range from 0.1 GHz to above 65 GHz with an on-chip footprint on the scale of a few μm. The frequency is inherent to the magnetic material of the NC-STO and is excited by an electrical DC current by means of the spin torque transfer effect. Although the general operation is well understood, more detailed aspects such as a generally nonlinear frequency versus current relationship, mode-jumping and high device-to-device variability represent open questions. Further application-oriented questions are related to increasing the electrical output power through synchronization of multiple NC-STOs and integration with CMOS integrated circuits.

This thesis consists of an experimental part and a simulation part. Experimentally, for the frequency stability it is found that the slow but strong 1/f-type frequency fluctuations are related to the degree of nonlinearity and the presence of perturbing, unexcited modes. It is also found that the NC-STO can exhibit up to three propagating spin wave oscillation modes with different frequencies and can randomly jump between them. These findings were made possible through the development of a specialized microwave time-domain measurement circuit. Another instrumental achievement was made with synchrotron X-rays, where we image dynamically the magnetic internals of an operating NC-STO device and reveal a spin wave mode structure with a complexity significantly higher than the one predicted by the present theory.

In the simulations, we are able to reproduce the nonlinear current dependence by including spin wave-reflecting barriers in the nm-thick metallic, magnetic free layer. A physical model for the barriers is introduced in the form of metal grain boundaries with reduced magnetic exchange coupling. Using the experimentally measured average grain size of 30 nm, the spin wave mode structure resulting from the grain model is able to reproduce the experimentally found device nonlinearity and high device-to-device variability.

In conclusion, the results point out microscopic material grains in the metallic free layer as the reason behind the nonlinear frequency versus current behavior and multiple propagating spin wave modes and thereby as a source of device-to-device variability and frequency instability.

Abstract [sv]

Dagens snabba utveckling inom informationsteknik drivs på av ständigt växande informationsmängder och deras samhällsanvändning inom allt från resursoptimering till underhållning. Utvecklingen möjliggörs till stor del hårdvarumässigt av miniatyrisering och integrering av elektroniska komponenter samt trådlös kommunikation med allt större bandbredd och högre överföringshastighet. Det senare uppnås främst genom utnyttjande av högre radiofrekvenser i teknologiskt tidigare oåtkomliga delar av spektrumet. Frekvensutnyttjandet har det senaste årtiondet ökat markant i mikrovågsområdet med typiska frekvenser runt 2.4 GHz och 5.2-5.8 GHz.

I den spinntroniska oscillatorn (STO:n) möjliggörs frekvensgenerering i det breda området från 0.1 GHz upp till över 65 GHz av en komponent med mikrometerstorlek som kan integreras direkt i CMOS-mikrochip. Till skillnad från i konventionella radiokretsar med oscillatorer konstruerade av integrerade transistorer och spolar, genereras mikrovågsfrekvensen direkt i STO:ns magnetiska material och omvandlas därefter till en elektrisk signal genom komponentens magnetoresistans. Dessa materialegenskaper möjliggör ett tillgängligt frekvensband med extrem bredd i en och samma STO, som därtill kan frekvensmoduleras direkt genom sin styrström och på så sätt förenklar konstruktionen av sändarsystem. STO:ns icke-linjära egenskaper kan potentiellt också användas för att i en och samma komponent blanda ned mottagna mikrovågssignaler och på så sätt förenkla konstruktionen även av mikrovågsmottagare.

STO:ns signalegenskaper bestäms av det magnetiska materialets fysik i form av magnetiseringsdynamik driven av elektriskt genererade spinnströmmar. I denna avhandling studeras denna dynamik experimentellt med särskilt fokus på frekvensstabiliteten i den hittills mest stabila STO-typen; nanokontakts-STO:n. Genom mätningar i tidsdomän av STO:ns elektriska signaler runt 25 GHz har frekvensstabiliteten funnits hänga samman med den typ av icke-linjärt beteende som också funnits vara utmärkande för tillverkningsvariationen i komponenterna. Mikroskopiska undersökningar av materialet visar att en trolig källa till denna variation är den magnetiska metallens uppbyggnad i form av korn i storleksordningen 30 nm, och datorsimuleringar av en sådan materialstruktur har visats kunna reproducera de experimentella resultaten. Därtill har en metod utvecklats för att med röntgenstrålning direkt mäta de små, magnetiska mikrovågsrörelserna i materialet. Denna röntgenteknik möjliggör detaljerade experimentella studier av magnetiseringsdynamiken och kan användas för att verifiera och vidareutveckla den existerande teorin för mikrovågsspinntronik.

Sammantaget förs STO-teknologin genom denna studie ett steg närmare sina tänkbara samhällsbreda tillämpningar inom snabb, trådlös kommunikation för massproducerade produkter med integrerad sensor- och datorfunktionalitet.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. 91 p.
Series
TRITA-ICT, 2016:18
Keyword
spintronics, microwave oscillators, magnetization dynamics, spin waves, phase noise, device modelling, electrical characterization, X-ray microscopy, STXM, XMCD
National Category
Condensed Matter Physics Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Physics; Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-188546 (URN)978-91-7729-045-2 (ISBN)
Public defence
2016-09-02, Sal C, Electrum, Isafjordsgatan 22, Kista, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2009-4190Swedish Research Council, 2012-5372
Note

QC 20160620

Available from: 2016-06-20 Created: 2016-06-13 Last updated: 2016-06-20Bibliographically approved

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